Current methods to evaluate cardiovascular disease (CVD) risk are primarily based on assessments made in the resting state, but human circulation and metabolism evolved to respond to physiologic stress. The systemic response to exercise-induced perturbations may carry key prognostic information regarding cardiovascular health and reserve capacity. In particular, impaired cardiorespiratory fitness, representing low peak oxygen uptake, is a potent predictor of CVD outcomes across strata of risk. However, the extent to which impaired cardiorespiratory fitness reciprocally influences derangements in distinct metabolic pathways remains unclear. This application will combine two state-of-the-art techniques ? advanced cardiopulmonary exercise testing (CPET) with comprehensive gas exchange measures and high-throughput profiling of ~290 circulating metabolites (measured at rest and peak exercise) ? to deeply phenotype the metabolic responses to incremental exercise. We postulate that impaired cardiorespiratory fitness associates with discrete metabolite signatures, that these metabolite signatures relate to prevalent subclinical CVD traits and incident CVD outcomes, and that these signatures are modified by aerobic exercise training. In Aim 1, we will evaluate the metabolite signatures of impaired cardiorespiratory fitness in 3040 participants enrolled in the community- based Framingham Heart Study. We will also analyze the relations of these metabolite signatures to clinical risk factors, novel risk markers, subclinical disease measures, and exercise-related excursions in select pathway biomarkers. In Aim 2, we will examine the relations of these metabolite signatures to incident CVD and metabolic syndrome in the Framingham Heart Study, and to CVD hospitalization and death in 1040 patients in a hospital-based referral cohort. In Aim 3, we will investigate the effect of aerobic exercise training on longitudinal changes in metabolite signatures of impaired cardiorespiratory fitness and CPET gas exchange variables in three distinct and clinically relevant hospital-based samples (total N=60). The overarching goal of this proposal is to evaluate the premise that the metabolic responses to exercise provide incremental information regarding the transition from cardiometabolic risk factors to overt CVD. This research will be accomplished in the setting of a comprehensive career development program designed to provide Dr. Nayor, an early career investigator and cardiologist, with the skills needed to become an independent physician- scientist in cardiovascular medicine. His long-term career goal is to use in-depth characterization of physiologic exercise responses to identify early CVD phenotypes that will further our understanding of disease pathogenesis and enable discovery of novel targets for prevention. An outstanding mentoring team and an advisory committee of established scientists in the fields of exercise physiology, metabolite profiling, and advanced epidemiological and biostatistical methods will guide the candidate in his transition to scientific independence over the course of the award period.